But whereas this radial velocity technique is, for now, limited to ground-based observatories and can only observe one star at the time, the transit technique can be employed in space telescopes such as the French Corot (2006-14) and Nasa’s Kepler (2009-18) and Tess (2018-present) missions. Between them, space telescopes have detected thousands of exoplanets in all their diversity, taking advantage of the fact we can measure stellar brightness more easily from space, and for many stars at the same time.
Despite the differences in detection success rate, both techniques continue to be developed. Applying both can give the radius and mass of a planet, opening up many more avenues for studying its composition.
To estimate possible compositions of our discovered exoplanets, we start by making the simplified assumption that small planets are, like Earth, made up of a heavy iron-rich core, a lighter rocky mantle, some surface water and a small atmosphere.
Using our measurements of mass and radius, we can now model the different possible compositional layers and their respective thickness. This is still very much a work in progress, but the Universe is spoiling us with a wide variety of different planets. We’ve seen evidence of rocky worlds being torn apart and strange planetary arrangements that hint at past collisions. Planets have been found across our galaxy, from Sweeps-11b in its central regions (at nearly 28,000 light years away, one of the most distant ever discovered) to those orbiting our nearest stellar neighbour, Proxima Centauri, which is “only” 4.2 light years away.
Searching for ‘another Earth’
In early July 2013, I (Christopher Watson) was flying out to La Palma for my first “go” with our recently commissioned Harps-N spectrograph. Keen not to mess up, my laptop was awash with spreadsheets, charts, manuals, slides and other notes. Included was a three-page document I had just been sent entitled: “Special Instructions for ToO” (Target of Opportunity).
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